Sounds like something is non-original about your seat installation.
I imagine you have regular nuts on the bottom.
Originally there were 4 steel bars on the bottom with 4 threaded holes each, 2 for flat head screws to hold it to the floor and 2 for the seat runners.
If you find regular nuts under there, I would put some vise grips on them to hold them while you unscrew the screws from the top side, or vise grips on the heads while you undo the nuts.
If that doesn’t work, grind off the screw heads or the nuts with an air angle die grinder and a 2" or 3" cutting wheel.